In the real estate industry, a need exists for controlled access to homes for sale that is both flexible to serve the real estate professional and secure for the homeowner's peace of mind. The traditional method has been the use of a key safe or lock box that attaches to the homeowner's doorknob and contains the dwelling key. Many conventional designs ranging from mechanical to electronic have been used over the years to provide this functionality. Homeowners prefer electronic systems because, unlike their mechanical counterparts, the electronic systems offer greater security and control over whom has access to the dwelling key and further offers the ability to track accesses to the key.
One challenge in previous designs has been the management and updating of electronic keys and electronic lock boxes with current access code information. The distribution of such information is compounded geometrically with the number of lock boxes and keys. This has not been a huge problem from the “key side” with the advent of central computer systems communicating with electronic keys; however, conventional systems now in use have not addressed the fundamental problem of updating lock box devices that are dispersed over a large geographic area. The previous designs and prior art patent literature provide an updating function via a radio signal or a pager; however, these systems are impractical due to the receiving circuit's power drain and potential proximity constraints with respect to the physical locations of receiver and transmitter.
Moreover, the convention electronic lock box systems have focused on loading electronic keys with access codes for use with lock boxes that could potentially be visited. In fact, these prior art systems have increasingly encompassed more costly and cumbersome electronic key solutions that are required to be periodically updated with new access codes.
Even with the more costly electronic key systems presently available, some convention card-based lock access control systems can be defeated by modifying the expiration data. Such electronic key systems have been in use for a number of years, particularly in the real estate industry. One example of such a design is disclosed in U.S. Pat. No. 4,988,987 (by Barrett et al.) which uses an expiration date in an electronic key and a calendar means in an electronic lockbox to enforce the actual expiration of an electronic key used by a real estate agent. The Barrett system compares the electronic key's transmitted expiration date with the calendar date contained in the electronic lock box.
It is known that such simple controls can be thwarted by an unscrupulous user by simply modifying the expiration data to a later date, thus eliminating the supposed benefit of an expiring right of access to a lockbox of Barrett's design, for example. Moreover, the expiration dates for some electronic key designs can be modified to create “immortal” keys, thereby potentially creating a “permanent” security hazard.
Convention lock box designs of course incorporate mechanical and electrical components that perform the functions of locking or releasing the door to a key compartment, and of locking or releasing the shackle that holds the lock box to a doorknob. There are occasions when the electromechanical components may become physically jammed, or perhaps frozen in place. If that has occurred and the lock box is actuated by a user, then the electric motor will not be able to move the mechanical unlatching components, and will endure “locked rotor” current. An overcurrent protective system has proved useful due to the relatively high current of the motor in this circumstance. Conventional lock box designs mainly have focused on utilizing motor overcurrent protection via additional current sensing electronics, or by use of self-resetting fuses that activate when the motor's stall current creates enough heat to open a motor “heater” (i.e., an overcurrent protection thermal switch). However, these conventional designs increase parts count and add complexity to their design.
It would be an improvement to provide a new method of access control of lock boxes using a simple to operate and manage system, using a new approach to the problem of access code synchronization between lock boxes and keys. Another improvement would be to provide a diversified numeric code transfer device that replaces conventional electronic keys, in which the diversified numeric code transfer device comprises a credit-card sized portable computer and a very thin secure memory card for a real estate agent for obtaining access to a lock box key compartment, in which the diversified numeric code represents a combination of the passage of time and a user's ID number. A further improvement would be to provide an electronic lock box that has a simplified mechanical construction for controlling access to a key compartment, and for controlling the release of the lock box from a fixed object, such as the door knob of a dwelling. Yet another improvement would be to detect a jammed or frozen mechanical component, and then prevent the motor from overheating by a system that does not require expensive overcurrent protection components (such as circuit breakers or thermal overload heaters).